← Waves

Pulse reflection: fixed end vs free end

A single pulse sent down a rope reflects DIFFERENTLY depending on how the far end is anchored — a distinct diagram-based skill from standing waves on a continuously driven string.

The two reflection rules

• FIXED END (rope tied to a wall or pole): the reflected pulse is INVERTED — a pulse that went up comes back down, because the fixed end cannot move and pushes back with an equal, opposite force.
• FREE END (rope end can move freely, e.g. attached to a ring that slides on a pole): the reflected pulse is UPRIGHT (not inverted) — the free end moves up further than the pulse's own height before springing back, reflecting the pulse the same way up.
• In BOTH cases, the pulse's SPEED and WAVELENGTH are unchanged by reflection — only the fixed-end case inverts the pulse's shape.

Worked example Drawing a pulse reflecting off a fixed end vs a free end The same upward pulse travels down two identical ropes — one tied to a fixed wall, one attached to a freely-sliding ring. Describe how each reflected pulse differs. FIXED END: the wall exerts an equal and opposite reaction force on the rope as the pulse arrives, flipping the pulse upside down — the reflected pulse travels back as a downward pulse. FREE END: the ring is free to move, so it is pulled UP by the arriving pulse, overshoots, then springs back down as it reflects — sending the pulse back UPRIGHT, the same way up as it arrived. BOTH CASES: the reflected pulse travels at the SAME speed as the incoming pulse (same rope, same tension and mass per length), just in the opposite direction. ✓ The key distinguishing word for each case is INVERTED (fixed end) vs UPRIGHT/NOT INVERTED (free end) — stating only "it reflects" without this distinction misses the actual physics being tested. ✕

Assuming every reflected pulse is inverted

Only a FIXED end inverts the reflected pulse. A free end reflects the pulse the SAME way up — assuming inversion always happens regardless of the end condition is the standard error this question type is designed to catch.

End type	Reflected pulse orientation
Fixed (tied to a wall/pole)	Inverted (flipped)
Free (can move, e.g. a sliding ring)	Upright (not inverted)

Practice for this standard covers the whole topic — flashcards and quiz.